# Adjoint Algorithm Design of Selective Mode Reflecting Metastructure for BAL Applications

## TL;DR

This paper introduces a new metasurface design for broad-area lasers to improve beam quality by selectively reflecting specific light modes.

## Contribution

A novel inverse design method using an adjoint algorithm for large-scale transverse mode-selective metastructures in BALs.

## Key findings

- The designed device achieved over 90% reflection for the fundamental mode.
- Higher-order transverse mode reflections were suppressed below 0.2%.
- The method was applied to the largest device structure designed using inverse methods.

## Abstract

Broad-area lasers (BALs) have found applications in a variety of crucial fields on account of their high output power and high energy transfer efficiency. However, they suffer from poor spatial beam quality due to multi-mode behavior along the waveguide transverse direction. In this paper, we propose a novel metasurface waveguide structure acting as a transverse mode selective back-reflector for BALs. In order to effectively inverse design such a structure, a digital adjoint algorithm is introduced to adapt the considerably large design area and the high degree of freedom. As a proof of the concept, a device structure with a design area of 40 × 20 μm2 is investigated. The simulation results exhibit high fundamental mode reflection (above 90%), while higher-order transverse mode reflections are suppressed below 0.2%. This is, to our knowledge, the largest device structure designed based on the inverse method. We exploited such a device and the method and further investigated the device’s robustness and feasibility of the inverse method. The results are elaborately discussed.